Get Bendy Bone base curve to match bezier curve
Found the answer from a BA thread asking the same question. I've tested this, and it works.

The length of each handle (hlength1 and hlength2 respectively) is based on the “ Ease In/Out ” properties, the length of the bone, and a magic-number factor (“0.5f * sqrt(2) * kappa”, where kappa = the handle length – apparently this formula allows “for near-perfect circles”). i.e.,

hlength1 = ease_in * length * 0.390464f hlength2 = ease_out * length * 0.390464f

From https://blenderartists.org/t/bendy-bone-that-matches-a-curve/1218163/13?u=oscalon, referencing https://code.blender.org/2016/05/an-in-depth-look-at-how-b-bones-work-including-details-of-the-new-bendy-bones/